As a technology for achieving a turbofan with less noise, for example, there is a structure disclosed in Patent Literature 1. A centrifugal fan disclosed in Patent Literature 1 includes an impeller including a main plate, a shroud, and a plurality of fan blades, a casing accommodating the impeller, and a suction bellmouth mounted to the casing. At a front edge portion of the fan blade, there is integrally formed a flat plate having the same thickness as that of the fan blade and a triangular shape. One side of the flat plate is held in close contact with the shroud at the front edge portion of the fan blade. With such a configuration, a flow on downstream of the suction bellmouth flows into the fan blade promptly and smoothly, and turbulence of the flow flowing into the fan blade is suppressed, thereby reducing noise.
Further, for example, in a centrifugal fan disclosed in Patent Literature 2, at an end (front edge portion) on an R direction side of a blade formed of a three dimensional blade, there is formed a front edge corner portion protruding toward an inner peripheral side of an impeller in a stepwise manner. The front edge corner portion is provided for an intention to obtain an effect of preventing an airflow from separating from a suction surface of the blade when the airflow sucked into the impeller through an inlet and a bellmouth is blown out to an outer peripheral side by the blade, thereby reducing noise of the fan.